Successful mounting of optics has been accomplished by various mechanical methods. Of these, bonded optic mountings can usually be effected more quickly and less-expensively than traditional mechanical clamping methods. The structural adhesives most frequently used to hold optics to mounts and to bond mechanical parts together are two-part epoxies, urethanes, and room-temperature-vulcanizing (RTV) elastomers. Adhesives emit volatile ingredients during cure or if exposed to vacuum or elevated temperatures. The emitted materials may then condense as contaminating films on nearby surfaces, such as optics, opto-mechanical assemblies, or items under process.
For all high vacuum applications, cross-linked polymers are most preferred because of extremely high molecular weights generated through the chemical reaction. Such polymers do not outgas except for trace amounts of residual components or unreacted low molecular species left in the polymer network. The two most frequently used polymers are epoxies and silicones. In some instances silicones are preferred over epoxies because of their ability to perform under extreme range of temperatures; however their adhesive properties with various surfaces are not as good as the epoxies. The epoxies and silicones typically used in current fabrication processes meet the users' current needs but may not necessarily meet the outgassing requirements set by the customer future generation optic assemblies (e.g., interferometers used in high vacuum environments).
For epoxy resins, proper formulation and mixing of the resin with the amine curing agent in stoichiometric proportions is critically important to meet the outgassing requirements under vacuum conditions ranging below 10−6 torr. Commonly used amine curing agents under ambient conditions have some vapor pressure and therefore a slight excess or improper mixing would present serious problems and would not meet the vacuum compatibility and outgassing specifications for the optic assembly. To address this potential issue, some epoxy formulations use amine adducts as curing agents. This approach is ideally suited for high vacuum applications. One distinct advantage of amine adduct curing agents is that it is a product of poly-functional amines and mono-functional epoxy intermediates and have much lower volatility due to higher molecular weight. As expected, this curing agent produces a superior vacuum compatibility epoxy end product with low-outgassing performance.
One area of concern is the outgassing of the materials used for optic assemblies in a vacuum environment. Typically the outgassing species include the following:                Trapped and dissolved gases, including nitrogen, oxygen and water vapor;        Possible solvents used the cleaning process and adsorbed gases;        Low molecular weight hydrocarbons and amines that were present as impurities in the epoxy resin and the curing agent; and        residual and unreacted epoxy and amine components.        
When building optic assemblies, optic adhesives are typically measured and mixed as needed. Assemblers are required to dispense an exact amount of the various constituents of two-part epoxies, urethanes, or RTVs. These constituents are then mixed, often in small quantities, then applied to the optic for mounting. The problem is getting exact adhesive mixing ratios with this method. And while this method is often adequate for many standard optic applications, newer applications have very tight requirements for outgassing and contamination.